The fabrication of semiconductor devices on semiconductor wafers requires that the semiconductor wafers be processed in a variety of different manners. Photolithography, chemical mechanical polishing (CMP), metal deposition, and so on, are all processes performed on semiconductor wafers in precise degrees to fabricate semiconductor devices. Usually, each of these different processes is performed using a different piece of semiconductor fabrication equipment. Therefore, a transport system is used to transport the semiconductor wafers among the different pieces of semiconductor fabrication equipment to achieve semiconductor device fabrication. Usually the semiconductor wafers are transported in pods.
FIG. 1 shows a top view of a traditional pod transport system 100. The system 100 particularly includes a conveyor belt 104 on which the pods may be transported among various stations 102a, 102b, . . . , 102n. Each of these stations 102a, 102b, . . . , 102n may be a separate piece of semiconductor fabrication equipment, a storage place to store pods and their semiconductor wafers, and so on. Overhead transfer (OHT) units 106 are able to move the pods vertically. The OHT units 106 are also referred to as OHT grippers, in that they grip the pods.
One particular area 108 of the system 100 of FIG. 1 is shown in more detail in a side view in FIG. 2. One of the OHT units 106 has a pod 202a that it is vertically lowering onto one of the load ports 204. One of the robot arms 208 has lowered on its vertical axis 206 to pick up the pod 202b that has already been lowered onto one of the load ports 204. The other of the robot arms 208 has risen on its vertical axis 206 to deposit the pod 202c onto one of the conveyor belts 104. In this way, pods 202 are moved from the OHT units 106 to the load ports 204, and ultimately to the conveyor belts 104. Furthermore, the OHT units 106 can vertically raise the pods 202 from the load ports 204.
A load port transfer system is used in conjunction with the load ports 204 to transfer wafer cassettes from the pods 202 to a process tool load position. The load port transfer system is more specifically a robotics load port transfer system. The transfer system operates under the control of a process tool computer, a host computer, or a user. FIG. 3 shows a side view of a load port transfer system 300 that is available from Asyst Technologies, Inc., of Fremont, Calif. The load port transfer system 300 specifically includes a lead screw mounting casting 302, a lead nut housing 304, a sensor rod 306, a lead screw 308, a vertical arm drive motor assembly 310, and a lower flag actuator 312.
The vertical arm drive motor assembly 310 controls the raising and the lowering of the lead screw 308, which can cause a platform 314 of the load port transfer system 300 to rise and descend. Wafer cassettes may be placed on the platform 314 for loading and unloading. The sensor rod 306 determines where the platform 314 is currently located. The lead screw 308 can have the associated lead screw mounting casting 302 and the leading nut housing 304. The lower flag actuator 312 may work in conjunction with the motor assembly 310 and/or the sensor rod 306.
A difficulty with the load port transfer system 300 is that the lead screw 308 may vibrate when the vertical arm drive motor assembly 310 is functioning. Such vibration may cause damage to the semiconductor wafers in the cassettes being transported in a worst-case scenario. However, at the very least, such vibration can cause the load port transfer system 300 to fail over time. That is, the vibration lessens the lifetime of the load port transfer system 300, and decreases its reliability.
A limited solution has been to use a ball-type lead screw as the lead screw 308. While this decreases vibration somewhat, vibration is still a problem. Therefore, there is a need to overcome these disadvantages associated with the load port transfer system 300. There is a need to further dampen vibration of the lead screw 308, even where the lead screw 308 is a ball-type lead screw. Such dampening should improve reliability and increase the operating life of the load port transfer system. For these and other reasons, therefore, there is a need for the present invention.